The present invention relates to the heat treating of elongated steel members, and, in particular, to a method for continuously leveling and quenching steel plates to provide required flatness without the need of post-quenching leveling and stretching operations.
The invention is particularly directed to the heat treatment of thin steel plates. As is commonly referenced, such plates have a thickness of less than 3/8 inch (0.95 cm.) and a width greater than 48 inches (122 cm.) and the invention will be described with particular reference thereto, although it will be hereinafter appreciated that the invention has applicability for widths and thicknesses therebeyond. The invention will also be described with reference to lower carbon alloy steels wherein the rate of quench is very high and residual longitudinal and lateral distortion preceding or following the quenching and/or tempering operations is particularly troublesome. Flatness has reference to the deviation of the top and bottom surface from a horizontal line when the plate is resting on a flat surface. The limit of the deviation is prescribed for varying thicknesses and width. While industry standards, such as ASTM Standard A-G Tables 16-17 is used to prescribe flatness tolerances, more often the customers or heat treaters will have more stringent requirements. All such standards recognize a greater permissible deviation as the plate gauge is reduced below 3/8 inch Flatness as used herein thus has prime reference to meeting or exceeding the standards applicable to the final product.
These lower carbon alloy steels, heat treated to improve their physical characteristics, are customarily hot rolled at temperatures between 2100.degree. F.-2500.degree. F. (927.degree. C.-980.degree. C.) in accordance with known procedures. The rolled plates, as received from the mill, are reheated to an elevated temperature of 1650.degree. F. (898.degree. C.) or above to assure complete austenitizing of the steel. Thereafter, the plates are subjected to a quenching operation wherein the plate is rapidly cooled from above its austenitizing temperature to below its Ms temperature without transformation. Subsequent thereto, the plates undergo a tempering operation whereby the residual stresses are relieved, and hardness and brittleness characteristics are modified and the ductility improved.
While such heat treating procedures readily provide the required physical properties, the ability to provide the required flatness specifications for certain applications is difficult to attain.
Flatness is a particular problem where the steel plate exiting the rolling operation is subject to longitudinal distortion. Therein, the longitudinal dimensions vary from side to side. Thus, if the edges are shorter than the middle portion, the plate will assume a bowed shape. Where the center dimension is shorter than the edge dimensions, the plate will assume a longitudinal lateral waviness. These physical distortions generally arise from processing conditions in the rolling operation. Furthermore, they may only be manifested after reheating in the austenitizing furnace and to a lesser extent after tempering. In both instances however, the cause for this distortion has been referenced to the rolling inaccuracies, chemical composition variations and temperature variations, all of which are interrelated.
Due to roll crown or center roller deflection under loading, the plate will oftentimes have a greater thickness at the center than at the edges of the plate. Due to differential cooling, this variation can additionally cause temperature variations across the width and attendent chemical composition variations. Each causes incremental stress variations and yield point variations which are physically manifested during the heating and cooling cycles. Where any of these variations cause uncorrected localized yielding, the ultimate product may be longitudinally and laterally distorted beyond acceptable specification.
Attempts have been made to remedy this post-rolling distortion by leveling intermediate the rolling and the heat treating operations. This has not, however, proven to be a particularly successful technique. While producing apparent leveling, the residual stresses cause the distortions to reappear during reheating. Further, the residual internal stresses may be manifested after heat treatment during shipping and other handling or temperature cylces.
Because of the high yield strength of the end product, generally in excess of 100,000 psi, post heat treating processing through leveling and stretching rolls is also of limited benefit in achieving flatness in the hardened plate.
It has been found that an acceptable flatness can be provided by controlling the conditions during the quenching operation. In my prior patent, U.S. Pat. No. 3,604,696, a continuous quenching operation is taught wherein uniformly and rapidly quenching the plate while continuously longitudinally and laterally restraining the plate during the cool down between a series of vertically aligned pressure rolls is effective to provide commercially acceptable flatness. This quench press produces controlled contraction and localized yielding of the plate during quenching thereby eliminating waviness and bowing present in the entering plate. Plates thicker than 3/8 inch, in particular, can be leveled with such an apparatus, with the flatness being retained or improved during the subsequent tempering.
Despite this in-treatment leveling, thinner plates oftentimes can not be effectively leveled. As was previously mentioned, post treatment leveling and stretching meets with limited success because of the high yield strength of the hardened plate. Thus, a plate with unacceptable flatness must be recycled through the heat treatment process, or failing that the plate may be designated for off-specification applications or scrap.